It has been proposed that working memory (WM) is updated/manipulated via a fronto-basal-ganglia circuit. One way that this could happen is via the synchronization of neural oscillations. A first step toward testing this hypothesis is to clearly establish a frontal scalp EEG signature of WM manipulation. Although many EEG studies have indeed revealed frontal EEG signatures for WM, especially in the theta frequency band (3-8 Hz), few of them required subjects to manipulate WM, and of those that did, none specifically tied the EEG signature to the manipulation process per se. Here we employed a WM manipulation task that has been shown with imaging to engage the prefrontal cortex and the striatum. We adapted this task to titrate the success of WM manipulation to approximately 50 %. Using time-frequency analysis of EEG, we showed that theta power increased over frontal cortex for successful versus failed WM manipulation, specifically at the time of the manipulation event. This establishes a clear-cut EEG signature of WM manipulation. Future studies could employ this to test the fronto-basal-ganglia hypothesis of WM updating/manipulation.